The present invention relates to a semiconductor design technology, and more particularly, to a technology for monitoring a fuse of a redundancy circuit in an internal circuit of a semiconductor memory device.
With advance of high integration technologies in semiconductor memory devices, the number of memory cells and signal lines for a single semiconductor memory device are rapidly increasing. Since the memory cells and the signal lines are integrated within a finite space, line widths of the internal circuit become narrower and sizes of the memory cells become smaller.
For the above reasons, failure of the memory cells in the semiconductor memory devices increases, but redundancy circuits for repairing defective memory cells of the semiconductor memory devices make it possible to produce the memory devices having expected capacity with high yield, in spite of defective cells. The redundancy cell includes fuses for programming repair addresses to redundancy memory cells and defective memory cells.
When a wafer process is completed, a variety of tests are carried out. When the memory cells determined as defective can be repaired, the defects are repaired by replacing them as redundancy memory cells. That is, the internal circuit performs a programming operation to replace an address of a defective memory cell with an address of a redundancy memory cell. Therefore, when the address of the defective memory cell is inputted, the defective memory cell is replaced with the redundancy memory cell, so that a normal operation is carried out.
A fuse programming is used to program the address information of the defective memory cell. A typical method is a laser blowing-type method, a so-called physical fuse type method, which blows a fuse by using a laser beam. However, the physical fuse type method using laser beam can be used only at a wafer level which is a step prior to fabrication of the semiconductor memory device into a package.
Therefore, instead of the physical fuse type method using laser beam, an electrical fuse type method is used to replace the defective memory cells at the package level. A fuse which is programmable at the package level is generically called an electrical fuse. This means that the programming can be performed by electrically changing the connection state of the fuse. Such an electrical fuse may be subdivided into an anti-type fuse, which changes an open state to a short state, and a blowing-type fuse, which changes a short state to an open state. The electrical fuse has a very high effectiveness because the programming is carried out after the packaging.
However, since the electric fuse type method is carried out at the package level, it is impossible to visibly monitor the connection state of the programmed fuse, as opposed to the physical fuse type method which is carried out at the wafer level. According to the related art, the package must be removed in order to monitor the connection state of the fuse programmed by the electrical method. However, the removal of the completed package in order for the test will lower the value of the completed product and reduce the efficiency of the test.